1. Technical Field
The present invention relates to a fog removing device, and in particular relates to a wide range of fog removing techniques.
2. Related Art
Japanese Patent Application Publication No. 2012-168936 discloses a fog removing technique based on an atmospheric model (please see the section RELATED ART). An atmospheric model is an optical principle which is utilized when an image of an object is captured by an image-capturing apparatus or an object is observed with naked eyes when airborne particles are present in an atmosphere. An atmospheric model is expressed with Equation (1).I(x)=J(x)t(x)+A(1−t(x))  Equation (1)
I denotes an observation image (image including fog), J denotes a processed image (image not including fog), t denotes the fog density, x denotes the coordinate of a target pixel and A denotes airglow.
An image I(x) observed by an image-capturing apparatus is configured with: a portion J(x)t(x) including reflected light J(x) from an object remaining even after being scattered by airborne particles in the air; and atmospheric light A(1−t(x)) which has resulted from sunlight having been scattered by the airborne particles in the air.
FIG. 10 of the above-mentioned Japanese patent application publication provides an explanation using an image having respective values of I(x), J(x), A and t(x).
By obtaining the fog density t and the airglow A with Equation (1), the processed image J can be obtained from the observation image I.
Kaiming He et. al, proposed a novel fog removing technique in their paper “Single Image Haze Removal Using Dark Channel Prior” at IEEE Conference on Computer Vision and Pattern Recognition, 2009.
To explain it simply, assuming in the atmospheric model that the minimum value of pixel values of each pixel and surrounding pixels thereof, which minimum value is obtained with Equation (2) (Dark Channel Prior, abbreviated to DCP hereinafter), represents the fog density, the degree of fog removal is changed according to the value of the DCP.
                              DCP          ⁡                      (            x            )                          =                              min                                          c                ∈                r                            ,              g              ,              b                                ⁢                      (                                          min                                  y                  ∈                                      Ω                    ⁡                                          (                      x                      )                                                                                  ⁢                              (                                                      I                    C                                    ⁡                                      (                    y                    )                                                  )                                      )                                              [                  Equation          ⁢                                          ⁢          1                ]            
This provides the DCP value of approximately 0 because a general natural image not including fog has a shadow or a portion with a high chroma around any pixel. On the other hand, in an image including fog, the luminance is raised due to the fog, and this raises the DCP value. Accordingly, fog removal becomes possible by performing a process so as to make the DCP value 0.
However, the above-mentioned fog removal approach has the following drawbacks. A densely foggy image has few color components, and no undulation. In such an image, the value of DCP and an input image become close, and performing a process so as to make the DCP value 0 results in a very dark processed image. Conversely, setting low parameters to prevent an image from becoming dark lessens the fog removal effect, and presents a drawback of not being able to make the image clear.
An object of the present invention is to solve the above-mentioned drawbacks and to provide a fog removing device to perform fog removal from an image including fog.